1. Field of the Invention
This invention relates to the field of optoelectronics, which includes light emitting components, such as light emitting diodes (LED) and laser diodes, and which also includes light detecting components, such as photodiodes, phototransistors, photodarlingtons and photovoltaic cells. Optoelectronics also includes various devices which incorporate optoelectronic components, such as displays, photosensors, optocouplers, and fiberoptic transmitters and receivers. In particular, this invention relates to lenses to increase the efficiency of optoelectronic emitters and the sensitivity of optoelectronic detectors.
2. Description of the Related Art
A prior art LED 100 is shown in FIG. 1 and consists of a semiconductor diode element 110 electrically connected to a leadframe 120 and surrounded by an encapsulating material 130. The diode element 110 is typically mounted to one lead 122 of the leadframe 120 and connected to a second lead 124 of the leadframe 120 by a wire bond 140. These two leads provide an electrical connection between an external current source and the anode and cathode of the diode element 110. The external current source supplies power to the diode device 100 that is converted to emitted light by the photoelectric effect, which occurs at the semiconductor junction within the diode element 110.
Internal inefficiencies within a semiconductor diode result in very low net efficiencies, which is the ratio of emitted light power to input power. Internal inefficiencies arise from a low ratio of minority carriers injected into the diode semiconductor junction to photons generated at the junction; photon loss due to internal reflection at the semiconductor/encapsulant interface; and absorption of photons within the semiconductor material. Because of these low net efficiencies, many LED applications require high input current, resulting in heat dissipation and device degradation problems in order to obtain sufficient light.
As illustrated in FIG. 1, the encapsulant 130 forms a flat light-transmitting surface 150. A flat surface is convenient in many applications where the LED is mounted to another surface that is also generally flat or in applications that otherwise cannot accommodate a protruding surface. The inefficiencies described above, however, are compounded by the configuration of the LED encapsulant/air interface. An encapsulant having a flat surface, such as in FIG. 1, allows photons transmitted by the diode element 110 to have considerable dispersion. A flat encapsulant surface also results in internal reflection at the encapsulant/air interface, further reducing photon transmission and increasing photon absorption within the encapsulant material.
FIG. 2 illustrates a prior art LED 200 having an encapsulant 230 that forms a spherical surface 250. A spherical or other curved surface gives a larger angle of incidence for photons emitted from the semiconductor diode element 210, reducing losses due to internal reflection. Further, this surface 250 acts as a lens to reduce the dispersion of generated photons. Unfortunately, a protrusion, such as this curved surface, is difficult to accommodate in many applications.
An optoelectronic device according to the present invention incorporates a lens that increases component performance. For example, the output of an LED utilizing the lens is increased by, in part, reducing internal reflection. Internal reflection results from the differing indices of refraction at the interface between the LED encapsulant and the surrounding air.
As shown in FIG. 3, when a light ray 310 passes from a media having a higher index of refraction 320 to a media having a lower index of refraction 330, the ray 310 is refracted away from the normal 340 to the surface 350. The angle, xcex81, is customarily referred to as the angle of incidence 370 and the angle xcex82 is customarily referred to as the angle of refraction 380. As the angle of incidence 370 is increased, the angle of refraction 380 increases at a greater rate, in accordance with Snell""s Law:
sin xcex82=(N1/N2) sin xcex81,
where (N1 greater than N2). When the angle of incidence 370 reaches a value such that sin xcex81=N2/N1, then sin xcex82=1.0 and xcex82=90xc2x0. At this point none of the light is transmitted through the surface 350, the ray 310 is totally reflected back into the denser medium 320, as is any ray which makes a greater angle to the normal 340. The angle at which total reflection occurs:
xcex8c=arcsin N2/N1
is referred to as the critical angle. For an ordinary air-glass surface, where the index of refraction is 1.5, the critical angle is about 42xc2x0. For an index of 1.7, the critical angle is near 36xc2x0. For an index of 2.0, the critical angle is about 30xc2x0. For an index of 4.0, the critical angle is about 14.5xc2x0.
An optoelectronic device according to the present invention has an encapsulant that functions as a lens. For emitter applications, the lens reduces internal reflection and dispersion without having a protruding curved surface. Thus, LEDs utilizing the present invention have an improved efficiency compared with prior art flat-surfaced LEDs and similar devices, without the physical interface difficulties of the prior art curved-surface LEDs and similar devices. For detector applications, the lens focuses photons on the active area of the detector, increasing detector sensitivity. This increased detector sensitivity allows a detector having a reduced size, hence a reduced cost, to be used for a given application.
A particularly advantageous application of an optoelectronic device with a non-protruding lens is in pulse oximetry, and in particular, as an emitter in pulse oximetry probes. Pulse oximetry is the noninvasive measurement of the oxygen saturation level of arterial blood. Early detection of low blood oxygen saturation is critical because an insufficient supply of oxygen can result in brain damage and death in a matter of minutes. The use of pulse oximetry in operating rooms and critical care settings is widely accepted.
A pulse oximetry probe is a sensor having a photodiode which detects light projected through a capillary bed by, typically, red and infrared LED emitters. The probe is attached to a finger, for example, and connected to an instrument that measures oxygen saturation by computing the differential absorption of these two light wavelengths after transmission through the finger. The pulse oximetry instrument alternately activates the LED emitters then reads voltages indicating the resulting intensities detected at the photodiode. A ratio of detected intensities is calculated, and an arterial oxygen saturation value is empirically determined based on the ratio obtained:
Ird/Iir=Ratioxe2x86x92% O2 Saturation
Typically, a look up table or the like correlates the Ratio to saturation. The use of conventional LEDs within pulse oximetry probes has a number of drawbacks. Pulse oximetry performance is limited by signal-to-noise ratio which, in turn, is improved by high light output emitters. LEDs without lenses, such as illustrated in FIG. 1, are not optimized to transmit the maximum amount of light into the skin. LEDs with protruding lenses, such as illustrated in FIG. 2, create increased pressure on the skin, resulting in perfusion necrosis, i.e. a reduction of arterial blood flow, which is the medium to be measured. A solution to this problem in accordance with the present invention is an LED incorporating a non-protruding lens.
One aspect of the present invention is an optoelectronic device that comprises an encapsulant having a surface, a lens portion of the surface, and a filler portion having a generally planar surface. The filler portion is disposed around the lens, and the lens does not extend substantially beyond the plane of the generally planar surface. The optoelectronic device also comprises an optoelectronic element embedded in the encapsulant and operable at at least one wavelength of light. The lens being configured to transmit or receive the at least one wavelength.
Another aspect of the present invention is a mold tool for an optoelectronic device that comprises a first mold piece having a surface that defines a first cavity and an aperture within the first cavity. The mold tool also comprises a second mold piece having a surface which defines a second cavity. The first cavity and second cavity cooperate to form a molding compound into a predetermined shape. The mold tool further comprises an ejector pin having a contoured tip. The pin is movably located within the aperture between a first position retracted within the cavity and a second position extended from the aperture. In the first position, the tip constitutes an integral portion of the first cavity. In the second position, the ejector pin facilitates removal of the compound from the first cavity. The ejector pin tip at least partially defines the predetermined shape.
Another aspect of the present invention is an optoelectronic method comprising the steps of providing a generally planar surface at a predefined distance from an optoelectronic element, defining a light transmissive region of that surface within the critical angle of the optoelectronic element, and contouring the surface within the transmissive region without exceeding the predefined distance. These steps create a non-protruding lens for the optoelectronic element. In one embodiment, the transmissive region has a circular cross-section. The optoelectronic method can comprise the further step of shaping a surrounding region adjacent said transmissive region.
Yet another aspect of the present invention is an optoelectronic device comprising an encapsulant means for embedding an optoelectronic element and a lens means for conveying light between the optoelectronic element and a media surrounding the encapsulant means. In one embodiment, the optoelectronic device further comprises a flat surface means for providing a low-pressure contact surface for the lens means. In that embodiment, the optoelectronic device can further comprise an arcuate surface means for avoiding total internal reflection of light from the flat surface means. In another embodiment, the optoelectronic device further comprises a surrounding surface means for providing a contact surface for the encapsulant from which the lens means does not protrude.